Micro controller units (‘MCU’) are widely used embedded in systems, such as telephones, clocks, appliances, and vehicles. Input and output devices for an MCU may be discrete switches, relays, or solenoids or other circuits. An MCU may lack any human interface devices at all and, for example, typically does not have keyboards, screens, disks, printers, or other such input/output devices of a personal computer. Micro controllers may control electric motors, relays or voltages, and may read switches, variable resistors, sensors or other electronic devices, for example.
Often, an MCU is a single integrated circuit, commonly with the following features:                one or more central processing units        discrete input and output bits, allowing control or detection of the logic state of an individual package pin        serial communications interfaces such as serial ports and Controller Area Network or other network interfaces for system interconnect        volatile memory (‘RAM’) for data storage and        Read-Only-Memory (‘ROM’) or Flash memory for program and operating parameter storage        
An MCU may also include:                peripherals such as timers, event counters, PWM generators        clock generator        analog-to-digital converters        in-circuit programming and debugging support        
Although great care is taken to ensure that MCUs are free from faults, in use they are still subject to internal and external faults and errors from causes that may be unrelated to the reliability of the MCU itself but may have external causes, such as electromagnetic interference or temperatures outside the specified operating range, as well as incorrect received signals or commands, for example. However, when an MCU is involved in a failure or error situation, and is returned to the supplier for examination, the original cause is often not present for diagnosis, especially if the fault condition was transient and the product has been removed from the environment where the fault occurred. Not only may this reflect unfairly on the reliability reputation of the product and its supplier, but also time and work is expended uselessly trying and failing to diagnose problems while the real cause goes undetected.
European patent specification 1 505 608 describes a memory system with an error detection interface relating to real time monitoring & collection of failures. Reports of the EASIS consortium on Electronic Architecture and System Engineering for Integrated Automotive Safety Systems describe a software provision for error tracking & logging.